1. Field of the Invention
The present invention relates to a capacity maintenance ratio determination device of a chargeable and dischargeable battery, a battery system and an electric vehicle.
2. Description of the Background Art
Degradation of a chargeable and dischargeable battery leads to its lower full charge capacity than its initial (when shipped from a factory, for example) full charge capacity. This causes discharge duration of the battery in a full charge state to be shortened as the degradation of the battery proceeds.
A capacity maintenance ratio, which is a ratio of a present capacity of the battery to a reference capacity, is used as a value representing a degradation level of such a battery. A larger capacity maintenance ratio indicates less advanced degradation of the battery, and a smaller capacity maintenance ratio indicates more advanced degradation of the battery.
The full charge capacity needs to be found in order to obtain the capacity maintenance ratio. JP 8-179018 A describes a remaining capacity display device that calculates the full charge capacity as a total capacity.
In the remaining capacity display device, first, the battery is charged for a given period of time, and a current flowing through the battery during the given period of time is integrated, thereby calculating an integrated capacity (an amount of electric charges) charged in the battery in the give period of time. In addition, a battery voltage is detected before and after the given period of time, and an amount of change in the battery voltage before and after the given period of time is calculated.
Then, the calculated integrated capacity is divided by the amount of change in the battery voltage, and the division result is multiplied by a difference value between the battery voltage in a complete discharge state and the battery voltage in the full charge state. The difference value is previously found by experiments and so on. The multiplication result found in the foregoing manner is displayed as the total capacity, that is, the full charge capacity of the battery. As described above, the full charge capacity of the battery is calculated assuming that the foregoing integrated capacity is proportional to the amount of change in the battery voltage in the remaining capacity display device of JP 8-179018 A.
In an actual battery, however, the foregoing integrated capacity is not exactly proportional to the amount of change in the battery voltage. Therefore, the full charge capacity of the battery cannot be accurately obtained in the remaining capacity display device described in JP 8-179018 A.
One method is considered for accurately finding the full charge capacity of the battery. In the method, the battery is completely discharged and then charged to the full charge state, and the full charge capacity is calculated based on a current integrated value during the charge.
This method, however, requires a sufficient period of time for charging/discharging the battery. In addition, when power failure or the like is taken into consideration, a battery used in a backup power supply, an electric vehicle and so on cannot be completely discharged.